Testing 3 wire Dometic Thermostat

Your Roof AC fan comes ON because you have the Tstat set to Auto. This is normal. Set Fan on Tstat to Low or High. Depending on HOW the OEM wired the AC and furnace 12 volt will determine what your problem is. DOES the roof AC function in all modes? It may be as simple as the furnace 12 volt fuse is blown. You have 2 blue wires at the upper AC control board. BOTH are related to the furnace Tstat operation. rv's will have 12 volts on one of those blue wires. Makes no difference which one. When you activate the furnace the AC board on top closes the furnace relay on that board and the 2nd BLUE wire now has 12 volts. Going back to the furnace to engage the furnace controls. IF you have NO 12 volts on either Blue wire the problem is between the Furnace and the upper AC control board and is NOT the wall tstat or AC control board. SOME Oems do NOT use the furnace tstat wires(furnace has 2, 12 volt OUT and then 12 volt IN from the AC board) for the 12 volt power to the upper AC control. Very rare but some do this. It would be fused at the main interior 12 volt fuse board. Doug